In most modern diesel-electric locomotives, the diesel engine drives the electric generators which in turn powers the electric motors that drive the locomotive wheels. The engine is typically a turbocharged diesel engine with turbochargers and aftercoolers. Every diesel electric locomotive has an engine cooling system.
The engine cooling system circulates the liquid coolant through the engine cooling loop to remove heat from the engine for two major reasons: (1) To keep the temperatures of the engine parts within allowable limits for reliability and durability and (2) to remove the heat from the incoming engine air (at the compressor output) to reduce the airbox air temperature which decrease the fuel consumption and reduce emissions.
In all present day engine cooling systems, a liquid coolant takes the heat from the engine (liners, heads, oil coolers, etc.), carries it to the radiators and discharges the heat to the surrounding air (or to sea water in marine applications). The coolant is usually a mixture of (a) water, (b) water-glycol solutions. There are two types of glycols used in these applications: (a) ethylene glycol and (b) propylene glycol. One of the characteristics of the glycols is their reduction of the freezing point of the water. Hence, the main purpose of using glycols is to reduce the freezing point of the coolant below the expected minimum temperature that the locomotive will encounter and thus reduce the freeze damage to components such as radiators. The higher the glycol percent, the lower the freezing point of the mixture. For example, the water freezes at 32° F., but the 50-50 mixture of the water and propylene glycol freezes at about −36° F. Hence, water-glycol mixtures are used extensively to protect freezing of the engine coolant at low ambient temperatures.
Locomotive operation requires special attention at very low ambient air temperatures. When the engine is operating at high loads, it transfers enough heat to the coolant so that there is no possibility for the coolant to freeze. On the other hand, if the heat transferred to the coolant is low and the ambient air temperature is low, there can be a possibility for the coolant to freeze. This is not desirable as it can create freeze-damage on components, particularly on radiators. Therefore, a number of special precautions are taken to prevent the freezing of the coolant as described hereafter.
A. Engine Idling: The engine may be run at an idle speed when the ambient temperature is low and the locomotive is not moving. This will keep the engine and coolant temperatures at a level that the engine can develop enough heat (and power) to keep the water temperatures above a safe minimum value. This alternative ensures the proper operation of the engine but has undesirable characteristics. First, idling consumes fuel even when the locomotive is not in use. In some business case studies, the cost of fuel consumed in idle operation for one year is estimated to be larger than the cost of developing alternative systems. Second, idling reduces the effective life of the engine.
B. Radiator Draining: When the engine is shut down, the water or coolant may be drained from the radiators completely to the water tank to eliminate freezing in the radiator tubes and damage them. This option requires large water tanks to hold the coolant volume in the radiators and connecting pipes. Almost all cooling systems that use water as coolant have this draining feature. This is commonly referred to as a “Dry Radiator” system. If the radiators are not drained, then it is referred to as a “Wet Radiator” system.
C. Layover System: In some locomotives, there is a system that is called the “Layover System”. This system enables shut down of the engine at cold ambient temperatures. Usually an electric heater (or other heat source) supplies the heat necessary to keep engine component temperatures at a minimum level so that the start-up of the engine is possible when desired.
D. Combined System: In another system, a combination of the above alternatives can be used. The following examples will be helpful in describing basic features of these alternative systems.
(1) Parking Locomotive Inside: With a dry radiator system, when the locomotive is parked inside a locomotive housing for overnight, the engine can be stopped. The coolant in the radiator is then drained and the engine components are kept at normal inside the building temperatures for start up the next morning. Parking the locomotive inside a heated building is limited by the available buildings. In most cases, it is not a practical solution.
(2) Parking Outside with Inside Heating: With a dry radiator system, the engine can be parked at outside, water is drained to the tank. At very cold nights, the engine coolant and oil temperatures can be lower than the engine start-up temperatures. So next morning, the locomotive is pulled and parked inside a heated building until temperatures reach up to start-up temperatures. This option also is not desirable by railroads as warming up the locomotive inside the building takes a long time. Moreover, a suitable building is not available in most locations.
(3) Start and Stop System: In this case, the locomotive is parked outside in cold weather. There is a system on the locomotive such that it automatically starts the engine when the coolant temperature goes below a predetermined level, and stops the engine when the coolant temperature reaches a maximum value. This way, the possibility of engine freeze is eliminated and the start-up of the engine is ensured the next day.
The start and stop alternative does not require any building or similar structure. It is part of the locomotive design and feature. However, it has two major drawbacks, namely, (a) it still requires the operation of the large locomotive engine (which is costly and reduces engine life), and (b) it is noisy and creates noise pollution. Starting and operating the locomotive engine at an urban environment, particularly during night hours, is restricted by local ordinances. Therefore, railroads specify certain conditions on layover systems precluding the start and stop option.
(4) Layover System with Dry Radiators (LSDR): With a dry radiator system, the engine is stopped but enough heat is supplied to coolant through a layover system (usually with an electric heater connected to an outside electric source). The coolant is circulating through engine and oil cooler but not through the radiators. This system is usually identified as the “Layover System with Dry Radiators.”
(5) Layover System with Wet Radiators (LSWR): With a wet radiator system, the engine is stopped but enough heat is supplied to coolant through a layover system as before. However, the coolant is circulating through the engine, oil cooler and the radiators. This system will be identified as the “Layover System with Wet Radiators.” In this case, the heat loss at the radiators will be higher than those of the LSDR system.
Before describing the proposed layover system, it is useful to describe the reasons for heating different engine and cooling system components. These are covered in this section. There are two major liquids used in locomotive diesels today. The engine coolant and engine oil. Any one or both of these liquids may be used to heat the engine during a layover period at low ambient air temperatures with forced or natural circulation modes.
Heating the engine oil is important for a number of reasons. The pour point of engine oils is high. As an example, the pouring point of SAE 40 oil is about −12° C. (or about 10.4° F.) (Ref: Material Safety Data Sheet # 1268, for Chevron Delo-6000 SAE 40 oil). If the oil temperature is permitted to go below this value, oil behaves like a soft plastic and will not flow. Therefore, it would not be possible to start the engine.
Moreover, the viscosity of oil goes up to a very high value at low temperatures, i.e., the viscosity of SAE 40 oil is 100 saybolds at 210° F. Corresponding values for 60 and 0° F. are about 7000 and 500,000 saybolds (Marks Mechanical Engineering Handbook, Sixth Edition, pp. 6–230, FIG. 1). The commonly recommended minimum oil temperature for engine start-up is about 40–50° F. Hence, heating the oil is a necessity for a reasonable sized, particularly an electric start-up system. The size, weight and cost of engine start-up systems go up very rapidly with decreasing start-up temperature.
Heating the oil directly with an electric heater has some limitations. As the heat conductance of the oil is low, the local temperature on the surface of the electric heater becomes very high. If this is permitted, it will start the oxidation of the oil even at low temperatures and consequently reduce the oil life to unacceptable levels. To prevent this oxidation, the heating rate (watt density) of the electric heater should be kept at a very low level. This in turn would increase the size of the electric heater necessary to do the job and become impractical. Hence, direct electric heating of oil is not utilized, but the engine coolant is heated by an electric heater, and the warm engine coolant transfers the necessary amount of heat to oil at the conventional oil cooler.
Heating the oil is usually done by forced circulation of the oil through the oil cooler and the engine. This will also assure proper lubrication as well as heating of surfaces that oil gets in contact with. When the engine is started, the bearings and the liner-ring interface already have the oil layer. This will reduce the power requirement for start-up, and the use of a smaller start-up system can be possible. Hence, oil heating is necessary to reduce the engine start-up power.
Forced convection of warm oil also heats the piston and the rings and therefore controls the clearance between the rings and piston at cold start conditions. This is important to bring the wear rate of the rings and liners. Hence, oil heating is also necessary for durability and reliability of the engine.
Heating the engine coolant is necessary for several reasons:
a) To control the proper clearance at engine liners. With decreasing ambient temperature, the liner will shrink and reduce the clearance between the liner and piston (rings). If the engine is started with liners that are at a temperature below a permissible low value, this will cause excessive wear and tear both on the rings and the liner. It will require a much higher start-up power and increase the size and cost of the starting system. It may also cause liner scuffing.
b) If the coolant is permitted to freeze, particularly within radiator tubes and liner passages, it may cause permanent damage to the tubes and other components.
c) At low enough temperatures, the water-glycol mixtures behave like a jelly and would not flow as easily. Hence, the coolant pump operation can be hindered at the start-up if the coolant temperatures are permitted to be too low.
d) The combustibility of the fuel injected into the engine cylinder depends on the air temperature in the cylinder. Heating the engine coolant will in turn heat the liner and through the liner, the air trapped in the cylinder. If the coolant is not heated, and at low ambient air temperatures, the fuel may not combust and starting the engine may not be possible.
e) At some low ambient temperatures, the fuel is not burned completely, leading to phenomena called “white smoke”. Heating the engine coolant tends to reduce and eliminate the engine white smoke and start-up emissions.
The heating of engine coolant and oil is necessary at low ambient air temperature conditions. An engine layover system is used to satisfy this need. At some applications where the ambient temperature becomes very cold, heating the locomotive cab also becomes an important issue for the crew. As a result, the locomotive cab heating system may be combined with the engine layover system to keep the engine as well as the locomotive cab temperatures within desirable limits.